A leading Corsican lawyer has been assassinated on his way to work, becoming the 15th victim this year of a spate of violence that police are linking to criminal gangs.

Antoine Sollacaro, one of the most prominent lawyers on the island, was shot while driving to work in the capital city of Ajaccio.

The 63-year-old, who was president of the bar association of Corsica, had defended a number of leading nationalists who have fought for independence from France and was a high-profile figure in the island's separatist movement.

He defended Yvan Colonna, who is serving life in jail for murdering the French prefect Claude Erignac, the highest state representative on the Mediterranean island, in 1998.

Sollacaro was pulling up in his Porsche at a garage near Ajaccio at 9am on Tuesday to buy a newspaper when the killers struck. His car was still moving as two men on a motorbike who had been following him approached the vehicle.

The passenger jumped off the motorbike and shot the lawyer six times in the head and neck with an automatic pistol, before jumping back on the bike and speeding off.

An hour before the murder, the body of Jean-Dominique Allegrini Simonetti, 50, a former nationalist militant, was discovered in mountains on the island.

Manuel Valls, the French interior minister, insisted law and order must be returned to the island after the killings and said he would visit Corsica in the next few weeks.

"The fight for security will be carried out with the greatest resolve," he said. "When the gowns of an advocate are attacked, it is a symbol of the challenge to the rule of law. The state will not forget the cruel price that its servants have paid [in Corsica]. It will mobilise to secure the end to this violence, something that is dearly wished by the vast majority of the population."

Many of the killings have been linked to a settling of scores between members of rival crime gangs, some of whom also have links with nationalist movements.

Sollacaro also defended some of the island's most notorious and feared criminal figures. Police are looking into his business and property dealings.

Jean-Marc Ayrault, the French prime minister, said the island's plummet into lawlessness was "intolerable". He added: "The government is determined to act against the plague of organised crime in Corsica."

Corsica, two-thirds of which is mountainous, is a popular holiday destination and the birthplace of the French emperor Napoleon Bonaparte. The northern town of Calvi also claims to be the birthplace of Christopher Columbus.

After hearing of the murder, Alain Pacoud, the president of the Human Rights League in southern Corsica, a position previously held by Sollacaro, told French journalists: "It's madness. Now we are not safe from anything. This could happen to anyone one day."

He added: "For a long time we thought this kind of thing was limited to the gangs, to the louts, but now they've killed a lawyer. Violence is poisoning the social body of the whole island."